The deposition of tungsten (W) films using chemical vapor deposition (CVD) techniques is an integral part of many semiconductor fabrication processes since it can produce low resistivity electrical connection between (i) adjacent metal layers (vias) and (ii) first metal layer and the devices on the silicon substrate (contact). In one tungsten deposition process, the wafer is heated to the process temperature in a vacuum chamber, and then a thin layer of tungsten film, known as the seed or nucleation layer, is deposited. Thereafter, the via or contact is filled with tungsten by the reduction of tungsten hexafluoride (WF6) by hydrogen (H2) (“plugfill” or the “bulk layer”). The bulk layer is generally deposited more rapidly than the nucleation layer, but cannot be produced easily and reliably without first forming the nucleation layer.
Various deposition methods can be used to form a thin tungsten nucleation layer. These include a chemical vapor deposition (CVD) and a pulsed nucleation layer (PNL) technique.
In the chemical vapor deposition (CVD) technique, the WF6 and reducing gas (e.g., SiH4 and/or H2) are simultaneously introduced into the reaction chamber. This produces a continuous chemical reaction of mix reactant gases that continuously forms tungsten film on the substrate surface. In a typical example, CVD nucleation layers are deposited from WF6-SiH4 with an Ar-H2 carrier gas. In some instances, CVD nucleation performance is enhanced by the presence of H2 in carrier gas mixture. Note that the WF6-SiH4 reaction is much faster than the WF6-H2 reaction due to lower activation energy and greater reactivity. WF6 will preferentially react with SiH4 and not start reacting with H2 until the SiH4 is gone.
In the pulsed nucleation layer deposition (PNL) technique, pulses of the reducing agent, purge gases, and metal-containing oxidizing agents are sequentially injected into and purged from the reaction chamber. The process is repeated in a cyclical fashion until the desired thickness is achieved. This process is described in U.S. patent application Ser. No. 09/975,074, previously incorporated by reference. PNL is similar to atomic layer deposition techniques reported in the literature. PNL is generally distinguished from ALD by its higher operating pressure range (greater than 1 Torr) and its higher growth rate per cycle (greater than 1 monolayer film growth per cycle). In the context of this invention, PNL broadly embodies any cyclical process of sequentially adding reactants for reaction on a semiconductor substrate. Thus, the concept embodies techniques conventionally referred to as ALD.
One important benefit of the PNL tungsten deposition process is a reduction in tungsten film roughness of up to ten times compared to tungsten grown with a CVD nucleation layer. Roughness is important for enhanced plugfill because a smooth film will create a smooth seam during the final stages of contact plugfill. Rougher tungsten films are undesirable because they tend to block closing seams with protruding tungsten grains and trap large void areas inside the feature. The seams produced during the final stage of plugfill can be exposed during tungsten chemical mechanical polishing (CMP) or etchback and result in significant integration problems for subsequent deposition steps such as copper barrier-seed and electroplating.
Other benefits of PNL tungsten deposition include (a) superior step coverage in comparison to CVD, (b) liner barrier insensitivity (PNL tungsten can integrate well with a large variety of Ti—TiN films), and (c) improved throughput in comparison to many conventional ALD processes due to multiple growth layers per cycle.
Improved methods for rapidly depositing smooth tungsten layers for plugfill would benefit the IC fabrication industry.